1. Field of the Invention
The present invention relates to an automatic buttonholer or buttonhole apparatus for use with a sewing machine and particularly, it relates to an automatic buttonholer designed to automatically determine the buttonhole size when the buttonhole is being stitched by a zigzag sewing machine.
2. Description of the Prior Art
In recent years, a sewing machine designed to oscillate its needle right and left for zigzag sewing has been put into practical use. Further, it has been proposed that such zigzag sewing machine be designed so that it is capable of stitching buttonholes. FIG. 1 shows an example of a buttonhole which forms the background of the present invention and which can be stitched by applying the present invention. Bar tacks BT1 and BT2 are formed at opposite ends of a buttonhole BH formed in cloth. Zigzag sewing or stitching having a predetermined amplitude l is applied to the left side SD1 and right side SD2 of the buttonhole BH. In this manner, the sewing of the buttonhole is performed.
In the sewing of such buttonhole shown in FIG. 1, it is necessary that its length L correspond with the size of a button to be inserted in the buttonhole BH. Heretofore, adjustments in machine operation, according to button hole size, have been made manually. However, in view of the troublesome nature of such manual operation, lately, it has been also proposed to replace manual operation by automatic operation. In an example of such automatic system, a cloth presser or shoe is provided with a button size sensing lever, the movement of which is transmitted to a buttonhole cam block by mechanical transmission means, e.g., a link mechanism. With the cam block connected to a cam shaft, the bar tacks BT1 and BT2 are sewn, whereupon the cam block is automatically separated from the cam shaft by the sensing lever and the left side SD1 and right side SD2 are then sewn.
However, in a conventional zigzag sewing machine of interest to the present invention, as described above, the presence of the link mechanism for transmitting the movement of the sensing lever which senses the button size results in a connection error or interference. Further, since it is necessary to directly transmit the movement of this lever to thereby switch the connection between the buttonhole cam block and the cam shaft, the switching position tends to be unstable and hence there has been a disadvantage that the resulting buttonholes are of uneven size. Further, in such systems having such a mechanical sensing lever, it is essential that the shoe and the sensing lever be integrally connected together so as to cause the sensing lever to follow the movement of the shoe. As a result, the shoe and the related parts become not only complicated in construction but also larger in size. Besides this, since the sensing lever is necessarily located near the area where the sewing needle operates, there has been a disadvantage that the sewing is troublesome; for example, the visual range is limited.